Articles containing copper nanoparticles and methods for production and use thereof

ABSTRACT

Articles containing a matrix material and plurality of copper nanoparticles in the matrix material that have been at least partially fused together are described. The copper nanoparticles are less than about 20 nm in size. Copper nanoparticles of this size become fused together at temperatures and pressures that are much lower than that of bulk copper. In general, the fusion temperatures decrease with increasing applied pressure and lowering of the size of the copper nanoparticles. The size of the copper nanoparticles can be varied by adjusting reaction conditions including, for example, surfactant systems, addition rates, and temperatures. Copper nanoparticles that have been at least partially fused together can form a thermally conductive percolation pathway in the matrix material.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.13/079,757, filed on Apr. 4, 2011 and incorporated herein by referencein its entirety.

This application is also related to U.S. patent applications Ser. No.12/512,315, filed on Jul. 30, 2009 and now available as U.S. Pat. No.8,105,414, and 12/813,463, filed on Jun. 10, 2010 and now available asU.S. Pat. No. 8,486,305. This application is also related toInternational Patent Application PCT/US2010/039069, filed on Jun. 17,2010. Each of these applications is also incorporated herein byreference in its entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not applicable.

FIELD OF THE INVENTION

The present invention generally relates to copper nanoparticles, and,more particularly, to copper nanoparticles contained in a matrixmaterial.

BACKGROUND

Nanoparticles can exhibit physical and chemical properties thatsometimes differ significantly from those observed in the bulk material.This is particularly true for copper nanoparticles, which can exhibit asignificantly reduced melting point relative to that of bulk copper. Inparticular, copper nanoparticles having a narrow size range andnanoparticle sizes of less than about 20 nm fuse together at much lowertemperatures and pressures than do larger copper nanoparticles or bulkcopper.

Although copper nanoparticles are of significant interest due, interalia, to the widespread industrial use of bulk copper, the formation ofmonodisperse copper nanoparticles remains synthetically challenging.Copper nanoparticles having a narrow size range that are less than about20 nm in size have been particularly difficult to synthesize.Solution-based chemical reduction methods have typically producednanoparticles having irregular shape, wide size ranges, and/ornanoparticle sizes that are much larger than 20 nm. Furthermore, manymethods for synthesizing copper nanoparticles are not readily amenableto scale up.

Only a limited number of scalable processes are available for producingmonodisperse copper nanoparticles having small nanoparticle sizes. Onereadily scalable procedure for synthesizing copper nanoparticles havingnanoparticle sizes below about 20 nm, more particularly below about 10nm, involves heating a copper salt solution, a bidentate diamine (e.g.,a N,N′-dialkylethylenediamine), and one or more C6-C18 alkylamines.Copper nanoparticles produced by this method have a fusion temperatureof less than about 200° C., with the fusion temperature decreasing as afunction of nanoparticle size. Copper nanoparticles in this size rangehave also been produced by the reduction of a copper salt in thepresence of ascorbic acid. Although copper nanoparticles in this sizerange can be isolated, characterized and utilized, they do have asomewhat limited shelf life. Further, rapid oxidation of copper can takeplace if the copper nanoparticles are incompletely fused after heating.

In view of the foregoing, facile utilization of copper nanoparticleshaving nanoparticle sizes of less than about 20 nm would be ofsubstantial benefit in the art. The present invention satisfies thisneed and provides related advantages as well.

SUMMARY

In various embodiments, articles containing a matrix material and aplurality of copper nanoparticles in the matrix material are describedherein. The copper nanoparticles are at least partially fused togetherand are less than about 20 nm in size.

In other various embodiments, compositions of the present disclosureinclude a plurality of copper nanoparticles that are less than about 20nm in size and further contain a surfactant system having a bidentatediamine and one or more C6-C18 alkylamines, and a matrix materialselected from the group consisting of a polymer matrix, a rubber matrix,a ceramic matrix, a metal matrix, and a glass matrix.

In various embodiments, methods of the present disclosure includeproviding a plurality of copper nanoparticles that are less than about20 nm in size, mixing the plurality of copper nanoparticles with amatrix material, and applying at least one of heat or pressure to atleast partially fuse the plurality of copper nanoparticles together.

In other various embodiments, methods of the present disclosure includeproviding an article containing a matrix material and a plurality ofcopper nanoparticles that have been at least partially fused together inthe matrix material, and placing the article in thermal contact with aheat source. The plurality of copper nanoparticles are less than about20 nm in size.

In still other various embodiments, methods of the present disclosureinclude providing a plurality of copper nanoparticles that are mixedwith a matrix material to form a paste, placing the paste in a jointbetween a first member and a second member, and joining the first memberto the second member by at least partially fusing the plurality ofcopper nanoparticles together. The plurality of copper nanoparticles areless than about 20 nm in size and further contain a surfactant systemhaving a bidentate diamine and one or more C6-C18 alkylamines.

The foregoing has outlined rather broadly the features of the presentdisclosure in order that the detailed description that follows can bebetter understood. Additional features and advantages of the disclosurewill be described hereinafter, which form the subject of the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present disclosure, and theadvantages thereof, reference is now made to the following descriptionsto be taken in conjunction with the accompanying drawings describing aspecific embodiments of the disclosure, wherein:

FIG. 1 shows an illustrative SEM image of copper nanoparticles;

FIG. 2 shows an illustrative EDS spectrum of copper nanoparticles;

FIG. 3 shows an illustrative XRD spectrum of copper nanoparticles;

FIG. 4 shows an illustrative SEM image of a micron-size copper crystalformed by fusion of copper nanoparticles at room temperature duringcentrifugation;

FIGS. 5A and 5B show illustrative SEM images of a network ofsubstantially fused copper nanoparticles;

FIG. 6 shows an illustrative close up SEM image of partially fusedcopper nanoparticles, which demonstrates widespread necking of theindividual nanoparticles; and

FIG. 7 shows an illustrative EDS spectrum of substantially fused coppernanoparticles.

DETAILED DESCRIPTION

The present disclosure is directed, in part, to articles containing amatrix material and a plurality of copper nanoparticles that have beenat least partially fused together, where the copper nanoparticles areless than about 20 nm in size. The copper nanoparticles are operable tobecome at least partially fused together by applying pressure and/orgentle heating (e.g., <˜200° C.). The conditions under which the coppernanoparticles become at least partially fused together generally do notdeform or otherwise damage the articles in which the coppernanoparticles reside. In fact, the copper nanoparticles can become atleast partially fused together during formation of the article (e.g.,during polymer curing, during extrusion, or during press molding of agreen material containing copper nanoparticles). Furthermore, the matrixmaterial lengthens the shelf life of the copper nanoparticles andimproves their stability against oxidation. Still further, upon becomingat least partially fused together, the copper nanoparticles can form anelectrically or thermally conductive percolation pathway in the matrixmaterial. This feature can allow an initially non-conductive matrixmaterial to become electrically or thermally conductive.

The present disclosure is also directed, in part, to methods for atleast partially fusing copper nanoparticles together in a matrixmaterial. These methods can further be used to facilitate thermaltransport in a matrix material and to join a first member and a secondmember together. In addition, fusible compositions containing aplurality of copper nanoparticles and a matrix material are describedherein.

In addition to the foregoing, it is believed that copper nanoparticlescan be used in various catalytic processes in which bulk coppercatalysts can be used. Such chemical processes can include, for example,the water-gas shift reaction and cross-coupling reactions (e.g., Glasercouplings, Suzuki-Miyaura couplings of boronates and vinyl or arylhalides and Ullmann couplings). Copper catalysts are also of particularimportance in the formation of silicones, where there is an ongoingsearch for new copper catalysts that can produce different types ofmaterials in higher yield and purity. Further, copper nanoparticles candisplay useful optical and electrical properties. As such, they havepotential uses in microelectromechanical and nanoelectromechanicaldevices, biomaterials, biomarkers, diagnostic imaging devices, sensors,coatings, composite materials, textiles, fuel cells, and solar cells.Still further, they can have potential applications in anti-microbialand anti-fungal compositions. Any of these applications can benefit fromplacing the copper nanoparticles in a matrix material, which functionsas a solid support.

As used herein, the term “size range” refers to the distribution ofnanoparticle sizes in a plurality of nanoparticles such that>95% of thenanoparticles have a size residing within the indicated size range.

As used herein, the term “average size” refers to the arithmetic mean ofthe distribution of nanoparticle sizes in a plurality of nanoparticles.

As used herein, the term “maximum size” refers to the largestnanoparticle size observed in a plurality of nanoparticles.

As used herein, the terms “fuse,” “fused” or “fusion” refer to acoalescence or partial coalescence between two or more nanoparticles. Inthe coalescence or partial coalescence of two or more nanoparticlesthere is necking and formation of a bond between the two or morenanoparticles. At or above the fusion temperature, the atoms on thesurface region of the nanoparticle behave as if that part of thenanoparticle were in the liquid state.

As used herein, the term “fusion temperature” refers to the temperatureat which a nanoparticle liquefies, giving the appearance of melting.

As used herein, the term “copper salt” refers to any salt of copper inany of its common oxidations states, including cuprous salts, i.e.,Cu(I), and cupric salts, i.e., Cu(II).

As used herein, the term “organic solvent” generally refers to polaraprotic organic solvents. Useful organic solvents of the embodimentsdescribed herein are capable of solubilizing copper salts and reducingagents or acting as co-solvents to solubilize copper salts and reducingagents.

In various embodiments, articles containing a matrix material and aplurality of copper nanoparticles in the matrix material are describedherein. The copper nanoparticles are at least partially fused togetherand are less than about 20 nm in size.

In some embodiments, the plurality of copper nanoparticles can furthercontain a surfactant system. Without being bound by theory or mechanism,it is believed that the surfactant system helps stabilize the coppernanoparticles after their formation and inhibits their agglomerationback into bulk copper. Surfactant systems suitable for synthesizingcopper nanoparticles are described in co-pending U.S. patentapplications Ser. No. 12/512,315, filed Jun. 30, 2009, and 12/813,463,filed Jun. 10, 2010, each of which is incorporated herein by referencein its entirety.

In some embodiments, the surfactant systems include, for example, aminecompounds or mixtures of amine compounds with a chelating agent. In someembodiments, the chelating agent is a bidentate diamine. In someembodiments, the bidentate diamine has secondary and/or tertiaryterminal amino groups. In some embodiments, secondary or tertiaryterminal amino groups can be present in combination with a primary aminein a bidentate diamine. Illustrative bidentate diamine chelating agentsinclude, for example, ethylenediamine and derivatives thereof (e.g.,N,N′-dimethylethylenediamine, N,N′-diethylethylenediamine, andN,N′-di-tert-butylethylenediamine). Other illustrative bidentate diaminechelating agents can include, for example, methylenediamine,1,3-propylenediamine and like derivatives thereof. In other embodiments,multi-dentate amine chelating agents can be used. Illustrativemulti-dentate chelating agents can include, for example,diethylenetriamine, triethylenetetramine and tetraethylenepentamine.Other examples of chelating agents that can be useful for preparingcopper nanoparticles include, for example, ethylenediaminetetraaceticacid and derivatives thereof, and phosphonates

In some embodiments, the surfactant system remains in the matrixmaterial of the present articles following at least partial fusion ofthe copper nanoparticles. In other embodiments, the surfactant systemremains associated with the copper nanoparticles following their atleast partial fusion. In still other embodiments, the surfactant systemis partially or completely removed from the present articles followingat least partial fusion of the copper nanoparticles. One of ordinaryskill in the art will recognize that the physical and chemicalproperties of the components of the surfactant system will determine itsultimate disposition in the present articles.

In some embodiments, the surfactant system contains a bidentate diamineand one or more C6-C18 alkylamines, where C6-C18 refers to the number ofcarbons in the alkyl group. In some embodiments, the bidentate diamineis a C1-C4 N,N′-dialkylethylenediamine, a C1-C4N,N′-dialkylmethylenediamine or a C1-C4N,N′-dialkyl-1,3-propylenediamine, where C1-C4 refers to the number ofcarbons in the alkyl groups. In such embodiments, the alkyl groups canbe the same or different. Such surfactant systems can be operable toproduce copper nanoparticles having a nanoparticle size of less thanabout 10 nm under mild heating conditions (e.g., 30-80° C.) usinginexpensive copper salts, reducing agents and solvents, according to theembodiments described herein. As described herein, the size range of thecopper nanoparticles can be tuned by adjusting, for example, thereaction temperature, the reagent concentrations, and/or the reagentaddition rate. For example, in some embodiments, heating conditionsbetween about 30° C. to about 50° C. can be used to control the sizerange copper nanoparticles produced.

In some embodiments, the surfactant system used for synthesizing coppernanoparticles contains an N,N′-dialkylethylenediamine. In someembodiment, the surfactant system contains a C1-C4N,N′-dialkylethylenediamine, a C1-C4 N,N′-dialkylmethylenediamine or aC1-C4 N,N′-dialkyl-1,3-propylenediamine. Without being bound by theoryor mechanism, it is believed that such diamine compounds function asbidentate ligands that can effectively chelate copper ions at the twonitrogen atoms and stabilize the formation of small diameter coppernanoparticles. In some embodiments, the alkyl groups of the C1-C4N,N′-dialkylethylenediamine, C1-C4 N,N′-dialkylmethylenediamine or C1-C4N,N′-dialkyl-1,3-propylenediamine are the same, while in otherembodiments they are different. The C1-C4 alkyl groups include methyl,ethyl, propyl, and butyl groups, including normal chain or branchedalkyl groups such as, for example, isopropyl, isobutyl, sec-butyl, andtert-butyl groups. Other bidentate, tridentate, and polydentate ligandscan also be employed in the surfactant system in alternativeembodiments. In general, the bidentate, tridentate or polydentateligands of the surfactant system are present in an amount ranging fromabout 12 percent to about 16 percent by volume of the reaction mixtureused for synthesizing copper nanoparticles, after addition of allreagents thereto. However, concentrations outside this range can also beused, if desired.

In some embodiments, the surfactant system also includes one or moreC6-C18 alkylamines. In some embodiments, the surfactant system includesa C7-C10 alkylamine. In other embodiments, the surfactant systemincludes a C11 or C12 alkylamine. In alternative embodiments, a C5 or C6alkylamine can be used instead of a C6-C18 alkylamine. One of ordinaryskill in the art will recognize that the size of the alkyl group in thealkylamine needs to be long enough to effectively form inverse micellesin the reaction mixture used for synthesizing copper nanoparticles,while maintaining sufficient volatility and being easily handled. Forexample, alkylamines having more than 18 carbons can also be used tosynthesize copper nanoparticles in alternative embodiments of thepresent disclosure, but they can be more difficult to handle because oftheir waxy character. In contrast, C7-C10 alkylamines provide a goodbalance of desired properties and easy use. In some embodiments, aC6-C18 alkylamine can be n-heptylamine. In other embodiments, a C6-C18alkylamine can be n-octylamine. In still other embodiments a C6-C18alkylamine can be n-nonylamine or n-decylamine. While these are allstraight chain amines, one of ordinary skill in the art will appreciatethat branched chain C6-C18 alkylamines (e.g., 7-methyloctylamine andlike branched chain alkylamines) can also be used.

Without being bound by theory or mechanism, monoalkylamines such asC6-C18 alkylamines also serve as ligands in the coordination sphere ofcopper ions, according to the various embodiments described herein.Unlike the bidentate diamines described above, however, themonoalkylamines more readily dissociate from the copper ions due totheir single point of ligation.

In general, the one or more C6-C18 alkylamines are present in an amountranging from about 10 percent to about 15 percent by volume of thereaction mixture used for synthesizing copper nanoparticles, afteraddition of all reagents thereto. However, alkylamine concentrationsoutside this range can also be used for synthesizing coppernanoparticles. Moreover, in some embodiments, the volume ratio of thebidentate, tridentate or polydentate ligand to the C6-C18 alkylamineranges between about 1:1 to about 2:1.

In some embodiments, methods for synthesizing copper nanoparticles usinga surfactant system that contains a C1-C4 N,N′-dialkylethylenediamineand a C6-C18 alkylamine includes at least the following operations: 1)heating a copper salt solution that contains a copper salt, an C1-C4N,N′-dialkylethylenediamine and a C6-C18 alkylamine in an organicsolvent to a temperature between about 30° C. and about 80° C.; 2)heating a reducing agent solution containing a reducing agent, anN,N′-dialkylethylenediamine and a C6-C18 alkylamine in an organicsolvent to a temperature between about 30° C. and about 80° C.; and 3)rapidly adding the heated copper salt solution to the heated reducingagent solution, thereby resulting in the production of coppernanoparticles. In some embodiments, heating can be conducted betweenabout 30° C. and about 45° C. or between about 30° C. and about 50° C.to better control the size range of the copper nanoparticles. In someembodiments, the heated reducing agent solution can be rapidly added tothe heated copper salt solution to result in the production of coppernanoparticles.

In some embodiments, the copper nanoparticles can be used in situwithout further isolation. In other embodiments, various workupprocedures can be performed to isolate and purify the coppernanoparticles. In some embodiments, these workup procedures can include,for example, rinses, sonication, centrifugation, repetitions thereof andcombinations thereof.

In carrying out the synthesis of copper nanoparticles according to thepresent embodiments, the heating of the copper salt solution and thereducing agent solution avoids uneven temperatures on mixing and allowsfor rapid addition of the two solutions. In some embodiments, rapidaddition means an addition that is completed in less than about 5minutes. In some embodiments, rapid addition means an addition that iscompleted in less than about 4 minutes, in less than about 3 minutes inother embodiments, in less than about 2 minutes in still otherembodiments, and in less than about 1 minute in still other embodiments.

In some embodiments, methods for synthesizing copper nanoparticlesutilize copper (I) and/or copper (II) salts. Illustrative copper saltsinclude, for example, copper halides, copper nitrate, copper acetate,copper sulfate, copper formate, and copper oxide. One of ordinary skillin the art will recognize the benefit of choosing a copper salt that hasgood solubility in the organic solvent being employed. Moreover, thechoice of copper salt can be a function of cost and scale. For example,inexpensive copper halide salts can be especially effective for largescale operations. Particularly, in some embodiments, the copper salt canbe a copper halide selected from copper chloride, copper bromide, orcopper iodide.

In general, a wide variety of reducing agents can be used in the presentmethods for synthesizing copper nanoparticles. Suitable reducing agentsare those that are compatible with the solvent being used and can reducecopper (II) to copper (0), copper (I), or mixtures thereof. In someembodiments, the reducing agent is a hydride-based reducing agent suchas, for example, sodium borohydride. One of ordinary skill in the artwill recognize that when employing a hydride-based reducing agent, thehydride source will provide the requisite change in copper oxidationstate. Although it is possible that copper hydrides initially form insome embodiments, they are believed to decompose rapidly to form copper(0).

In some embodiments, synthesis of copper nanoparticles is carried out inan organic solvent. The organic solvent can be substantially anhydrousin some embodiments. In some embodiments, the organic solvent can be apolar aprotic organic solvent that is capable of at least partiallysolubilizing the copper salt and the reducing agent. Illustrative polaraprotic organic solvents include, for example, N,N-dimethylformamide,dimethyl sulfoxide, dimethylpropylene urea, hexamethylphosphoramide,glyme, diglyme, triglyme, tetraglyme, tetrahydrofuran, 1,4-dioxane, and2,3-dimethoxy-1,4-dioxane. In some embodiments, the organic solvent istriglyme. This organic solvent provides good solubility for copperchloride and simultaneously activates sodium borohydride to function asa reducing agent. In some embodiments, the presence of the surfactantsystem in the organic solvent can also assist in dissolution of thecopper salt through the formation of a copper-organic ligand complex. Inaddition, co-solvents can be used to assist the dissolution of thecopper salt and/or the reducing agent.

The sizes of the copper nanoparticles synthesized by the above methodscan be sensitive to the temperature at which the reaction is conducted.In various embodiments, the sizes of the copper nanoparticles can rangesubstantially between about 1 nm and about 10 nm. When the reactiontemperature ranges between about 30° C. and about 45° C. or betweenabout 30° C. and about 50° C., the copper nanoparticles can generallyrange between about 1 nm and about 5 nm in size. However, when thereaction temperature ranges between about 45° C. and about 50° C. orbetween about 50° C. and about 65° C., the copper nanoparticles cangenerally range between about 5 nm and about 10 nm in size. Coppernanoparticles having other size ranges can be obtained through routineexperimentation.

Without being bound by theory or mechanism, it is believed that thetemperature control over these narrow ranges is facilitated by separateheating of the copper salt solution and the reducing agent solutionprior to mixing, thereby providing the resultant narrow distribution ofcopper nanoparticle sizes. In some embodiments, copper nanoparticlessynthesized by the present methods range between about 1 nm and about 10nm in size. In other embodiments, the copper nanoparticles range fromabout 1 nm to about 5 nm in size, or from about 2 nm to about 5 nm insize, or from 3 nm to about 5 nm in size. In still other embodiments,the copper nanoparticles range between about 5 nm and about 10 nm insize. In general, the copper nanoparticles have a substantiallyspherical shape. In some embodiments, copper nanoparticles that are lessthan about 20 nm in size can be produced.

As noted above, the fusion temperature can be a function of the size ofthe copper nanoparticles, with smaller copper nanoparticles having lowerfusion temperatures. In some embodiments, the fusion temperature of thecopper nanoparticles ranges between about 100° C. and about 200° C. Inother embodiments, the fusion temperature of the copper nanoparticles isless than about 100° C.

Unless otherwise noted, the fusion temperatures given herein are thoseobserved in the absence of an externally applied pressure (other thanatmospheric pressure). However, the fusion temperature is also afunction of applied pressure, with the fusion temperature decreasing athigher pressures. In some embodiments, sufficient pressure to induce atleast partial fusion of the copper nanoparticles can be imparted byprocesses such as ink jet printing, extrusion or centrifugation. Forexample, substantial fusion of copper nanoparticles at room temperaturecan occur at a pressure of as little as ˜3.9 atmospheres aftercentrifugation for 20-30 minutes, thereby producing micron size coppercrystals. Controlling the amount and duration of the applied pressurealso allows modulation of the degree of fusion of the coppernanoparticles. For example, application of 2000 psi static pressure tocopper nanoparticles produced 25% uniform porosity at 300° C. (morefusion) and a higher uniform porosity of 37% at 160° C. (less fusion).In sum, both the size of the copper nanoparticles and the pressureapplied thereto can be used to modulate the fusion temperature to adesired level.

In addition to the methods described above, copper nanoparticles canalso be synthesized by other methods known to those of ordinary skill inthe art. For example, copper nanoparticles having an average size ofabout 3.4 nm can be prepared via ascorbic acid-mediated reduction. Sucha copper nanoparticle synthesis is described in Wu, et al., “SimpleOne-Step Synthesis of Uniform Disperse Copper Nanoparticles,” Mater.Res. Soc. Symp. Proc., 879E:2005, pp. Z6.3.1-Z6.3.6. However, thesecopper nanoparticles lack the surfactant system referenced above.Instead, the copper nanoparticles synthesized by this method are coatedwith polyvinylpyrrolidone, which is much less easily removed than thepresent surfactant system.

In some embodiments, articles of the present disclosure contain coppernanoparticles that range between about 1 nm and about 10 nm in size. Inother embodiments, articles of the present disclosure contain coppernanoparticles that range between about 1 nm and about 5 nm in size orbetween about 5 nm and about 10 nm in size. In still other embodiments,articles of the present disclosure contain copper nanoparticles thatrange between about 1 nm and about 20 nm in size. As noted above,control over the size range of the copper nanoparticles can be modifiedthrough varying the reaction temperature, among other reactionparameters. In some embodiments, the size distribution of the coppernanoparticles is a narrow (e.g., ±about 1 nm) distribution. In otherembodiments, the size distribution of the copper nanoparticles is abroader distribution (e.g., ±about 2 nm to 4 nm or even greater). One ofordinary skill in the art will recognize that a narrow distribution anda wide distribution can have the same average nanoparticle size, if thearithmetic means of the nanoparticle size distributions are the same. Insome embodiments, the copper nanoparticles have a substantially Gaussiannanoparticle size distribution. In other embodiments, the coppernanoparticles have a nanoparticle size distribution that is non-Gaussianin nature.

In various embodiments, articles of the present disclosure containcopper nanoparticles that are at least partially fused together. As usedherein, a copper nanoparticle that is partially fused to other coppernanoparticles retains at least some of its original shape followingfusion. At a minimum, substantially spherical copper nanoparticles thatare at least partially fused together are at least tangential to oneanother. For example, two substantially spherical copper nanoparticlesthat are partially fused together can have a dumbbell or FIG. 8 shape.At greater degrees of nanoparticle fusion, the partially fusednanoparticles can acquire a shape that does not resemble that of theoriginal nanoparticles. For example, at greater degrees of nanoparticlefusion, two substantially spherical nanoparticles can acquire anellipsoid shape. In alternative embodiments, a plurality of coppernanoparticles that are at least partially fused together can have atleast some copper nanoparticles that are not fused with other coppernanoparticles.

In other embodiments, articles of the present disclosure can containcopper nanoparticles that have been completely or substantially fusedtogether. That is, the copper nanoparticles do not retain any of theiroriginal nanoparticle shape after being fused.

Copper nanoparticles offer a unique way of infiltrating the matrixmaterial of an article with copper. In some embodiments, the at leastpartially fused copper nanoparticles form an electrically or thermallyconductive percolation pathway in the present articles. In someembodiments, the at least partially fused copper nanoparticles arepresent in the matrix material in the form of a film or a layer withinthe article. In some embodiments, the entire article contains the atleast partially fused copper nanoparticles. In other embodiments, only aportion of the article contains the at least partially fused coppernanoparticles.

Whether the copper nanoparticles become partially or completely fusedtogether in the matrix material depends, inter alia, on the temperaturesand/or pressures to which the copper nanoparticles are exposed and theexposure time. As noted above, the present disclosure advantageouslydescribes articles containing copper nanoparticles of a size such thattheir fusion temperature is less than about 200° C., again depending onthe temperature and/or pressure applied thereto. In some embodiments,the article is heated at a temperature of less than about 200° C. inorder to at least partially fuse the copper nanoparticles. In otherembodiments, the article is heated at a temperature of less than about100° C. in order to at least partially fuse the copper nanoparticles.These fusion temperatures are much lower than that of bulk copper oreven larger nanoparticles, thereby allowing the infusion of copper intoarticles that would not withstand the temperatures required to melt bulkcopper.

One of ordinary skill in the art will further recognize that whether thecopper nanoparticles are partially or completely fused together in thepresent articles can also depend, at least to some degree, on theconcentration of copper nanoparticles contained within the matrixmaterial. For example, at lower copper nanoparticle concentrations, thecopper nanoparticles can be more well dispersed from one another,thereby leading to a lower likelihood of coalescence upon heating and/orapplying pressure. However, at higher nanoparticle concentrations,application of heat and/or pressure can lead to at least partialcoalescence between the copper nanoparticles simply by their being inclose contact with one another prior to fusion.

Further, the strength of the interaction (i.e., bond strength) betweenthe copper nanoparticles can be controlled by varying the degree ofcopper nanoparticle fusion. For example, the bond strength at theinterface between the copper nanoparticles is very strong, approachingthat of bulk copper, when the degree of fusion is complete or verynearly complete. In an embodiment, such strong bonding can occur whenthe copper nanoparticles fuse together to form a thermally conductivepercolation pathway. In contrast, when the degree of nanoparticle fusionis less than complete (i.e., partial fusion), the bond strength at theinterface between the copper nanoparticles is weaker. By way ofnon-limiting example, weaker bonding between the copper nanoparticlescan be desirable when the copper nanoparticles are distributed in anepoxy matrix. By way of further non-limiting example, an articlecontaining about 20% copper nanoparticles by weight in a flexiblepolymer (e.g., a rubber or silicone polymer) can maintain at least somedegree of flexibility. However, if the amount of copper nanoparticles isincreased to about 80% by weight or even higher, the article can becomesubstantially rigid, particularly after copper nanoparticle fusion

In some embodiments, articles of the present disclosure further includea filler material. Illustrative filler materials include, withoutlimitation, flame retardants, UV protective agents, antioxidants,graphite, graphite oxide, graphene, carbon nanotubes, fiber materials(e.g., carbon fibers, glass fibers, metal fibers, ceramic fibers andorganic fibers) and ceramic materials (e.g., silicon carbide, boroncarbide, boron nitride, and the like). In some embodiments, the fillermaterial is also in the nanoparticle size range.

UV protective agents that can be used as filler materials in the presentembodiments include, for example, organic, organometallic and inorganiccompounds that absorb light between about 200 and about 400 nm (e.g.,near- and middle ultraviolet light). Illustrative UV protective agentsinclude, for example, titanium dioxide, zinc oxide, stilbene andsubstituted stilbenes (e.g., TINOPAL LPW available from Ciba-GeigyCorp.), MEXORYL SX (ecamsule) which is a benzylidene camphor derivativeavailable from L'Oreal, oxybenzone, and avobenzone.

Antioxidants that can be used as filler materials in the presentembodiments include, for example, ascorbic acid, butylatedhydroxyanisole, butylated hydroxytoluene (BHT), gallate esters includingpropyl gallate, and α-tocopherol.

In various embodiments, matrix materials suitable for use in the presentarticles can be polymer matrices, including rubber matrices. In someembodiments, the polymer matrices are thermoplastic polymer matrices,thermosetting polymer matrices (i.e., epoxy matrices) or elastomericpolymer matrices (e.g., natural or synthetic rubbers). Illustrativepolymer matrices that can be used in the present embodiments include,for example, polycarbonantes, cyanoacrylates, silicone polymers,polyurethanes, natural and synthetic rubbers, derivatives thereof andthe like. In some embodiments, the copper nanoparticles are distributedin a two-component epoxy precursor system which is then cured. In otherembodiments, the copper nanoparticles are distributed in a softenedthermoplastic or elastomeric matrix, which is thereafter cooled andhardened to encapsulate the copper nanoparticles therein.

In some embodiments, the matrix material can be a silsesquioxane suchas, for example, POSS (Polyhedral Oligomeric Silsesquioxane), which iscommercially available from Reade Advanced Materials and has thefollowing structural formula.

In the indicated structural formula, the R groups can be the same ordifferent in various embodiments. In general, the R groups are alkylgroups, which can optionally contain further substitution. Illustrativeexamples of functional groups that can further substitute the R groupsin POSS include, for example, thiols, sulfides, amines, amides, hydroxylgroups, and carboxylic acids.

In some embodiments, the matrix material is a phase change material,particularly a phase change polymer, that isothermally releases orabsorbs heat upon changing state. Illustrative phase change materialssuitable for use in the present embodiments include, for example,paraffins (e.g., compounds having a formula C_(n)H_(2n+2), where n is aninteger) and fatty acids (e.g., compounds having a formulaCH₃(CH₂)_(2n)CO₂H, where n is an integer).

In alternative embodiments, the matrix material can be a material suchas, for example, ceramic materials, glasses and metals. For example,siloxanes and alumoxanes can be used as matrix materials. Coppernanoparticles can be particularly useful in polymeric, glass and ceramicmatrix materials, since at least partially fused nanoparticles can forman electrically or thermally conductive percolation pathway in thesenormally non-conductive materials.

In some embodiments, the matrix material (e.g., a polymer matrix, arubber matrix, a glass matrix, a ceramic matrix or a metal matrix)serves not only as a continuous phase to support the coppernanoparticles, but it also protects the copper nanoparticles fromoxidation. Particularly after being at least partially fused, coppernanoparticles can be especially susceptible to oxidation. The matrixmaterial can shield the at least partially fused copper nanoparticlesfrom an oxidizing environment, thereby slowing or substantially stoppingvarious oxidation processes by filling voids between the coppernanoparticles.

In some embodiments, the matrix material of the present articles isremovable. Thus, in some embodiments, the matrix material of the presentarticles can be removed to leave behind a network of at least partiallyfused copper nanoparticles. Depending on the degree of nanoparticlefusion and the concentration of copper nanoparticles initially presentin the matrix material, the network that remains behind can be acontinuous, essentially non-porous copper network or a copper networkhaving at least some degree of porosity.

In general, copper nanoparticles are present in the articles describedherein in an amount ranging between about 10% and about 99.9% of thearticle by weight. In some embodiments, the copper nanoparticles arepresent in an amount ranging between about 10% and about 50% by weightor, in other embodiments, in an amount ranging between about 20% andabout 60% by weight, or, in other embodiments, in an amount rangingbetween about 25% and about 50% by weight. In some embodiments, thecopper nanoparticles are present in an amount ranging between about 70%and about 99.9% of the article by weight. In other embodiments, thecopper nanoparticles are present in an amount ranging between about 80%and about 99% of the article by weight or, in still other embodiments,between about 90% and about 99% of the article by weight.

In some embodiments, the plurality of copper nanoparticles form athermally conductive percolation pathway in the matrix material of thepresent articles. Such a thermally conductive percolation pathway can beformed from partially fused copper nanoparticles or completely fusedcopper nanoparticles. In forming a thermally conductive percolationpathway using at least partially fused copper nanoparticles, articleshaving much higher thermal conductivities can be formed than when usingbulk copper. For example, in some embodiments, articles of the presentdisclosure that contain at least partially fused copper nanoparticleshave thermal conductivities ranging between about 50 watts/m·K and about400 watts/m·K. In contrast, like articles containing micron size bulkcopper particles typically have thermal conductivities in the range ofabout 5 watts/m·K to about 7 watts/m·K.

Due to their high thermal conductivities, the articles of the presentdisclosure can serve as a thermal interface material. An illustrativebut non-limiting use of the present articles in this capacity is as aheat transfer medium in thermal contact with a heat source and a heatsink. In particular, the present articles can be especially useful forfacilitating heat transfer from the central processing unit (CPU) ofcomputers and like electronic devices. The CPUs of modern computers andlike electronic devices put off significant quantities of heat, but theheat transference to the thermal ground plane or a heat dissipatingdevice such as a fan, for example, is typically poor. As a result,active cooling measures are often utilized. By serving as a thermalinterface material in contact with a heat source and a heat sink,articles of the present disclosure can overcome the heat transfer issuespresent in computers and other devices requiring transfer of excessheat. In some embodiments, the heat sink with which the present articlesare in thermal contact can be a source of cooling water, refrigeration,a fan, a radiator or like heat dispersal medium that is separate fromthe articles. In some embodiments, the present articles can beconstructed such that they dissipate excess heat themselves by havingcooling vanes, coolant circulation pathways and the like. Stated anotherway, the present articles can be constructed such that they both conductexcess heat away from a heat source and dissipate the excess heat to theatmosphere or other heat sink. In some embodiments, the articlessuitable for transferring heat away from a heat source include an epoxymatrix containing copper nanoparticles that have been at least partiallyfused together. In some embodiments, the epoxy matrix of such articlescan further include at least one of silver, aluminum, graphite, graphiteoxide, graphene, carbon nanotubes, fiber materials (e.g., chopped carbonfibers) or boron nitride. These materials can further aid the heattransfer.

The thermal conductivities of the present articles can be controlledthrough modulation of the amount of copper nanoparticles containedtherein, as discussed hereinafter. For applications taking advantage ofcopper's thermal conductivity, the present articles can contain betweenabout 10% and about 100% of copper nanoparticles by weight. The lowerend of this range will ultimately be determined to a large degree by theintended application and the degree of thermal conductivity required. Asnoted above, when the concentration of copper nanoparticles approaches100% by weight, the nanoparticle interface approaches that of bulkcopper, although the copper nanoparticle structure can also bemaintained, at least to some degree, in some embodiments, if onlypartial fusion has taken place. Regardless of the copper nanoparticleconcentration in the present articles, the thermal conductivities can bemuch higher than like articles containing micron size copper particles.For example, when the present articles contain between about 95% toabout 100% copper nanoparticles by weight, the thermal conductivitiescan approach 400 watts/m·K. On the lower end of the above concentrationrange, an article that contains about 15% to about 25% coppernanoparticles by weight can have a thermal conductivity of about 50-100watts/m·K. Thus, even at low copper nanoparticle concentrations, thethermal conductivities of the present articles are one to two orders ofmagnitude greater than that of typical thermal interface materials. Atabout 50% copper nanoparticles by weight, the present articles can havea thermal conductivity of about 200 watts/m·K.

In various embodiments, methods for using articles containing coppernanoparticles in heat transfer applications are contemplated herein. Invarious embodiments, the methods include providing an article containinga matrix material and a plurality of copper nanoparticles that have beenat least partially fused together in the matrix material, and placingthe article in thermal contact with a heat source. The plurality ofcopper nanoparticles are less than about 20 nm in size. In someembodiments, the methods further include placing the article in thermalcontact with a heat sink. In some embodiments, the plurality of coppernanoparticles further include a surfactant system having a bidentatediamine (e.g., a C1-C4 N,N′-dialkylethylenediamine, a C1-C4N,N′-dialkylmethylenediamine or a C1-C4N,N′-dialkyl-1,3-propylenediamine) and one or more C6-C18 alkylamines.As noted above, use of other bidentate, tridentate, or polydentateligands and alkylamines also lie within the spirit and scope of thepresent disclosure.

In some embodiments, the plurality of copper nanoparticles form athermally conductive percolation pathway in the matrix material afterbeing at least partially fused together. In some embodiments, thethermally conductive percolation pathway is such that the article has athermal conductivity ranging between about 50 watts/m·K and about 400watts/m·K. In some embodiments, the plurality of copper nanoparticlesare substantially non-porous after being at least partially fused. Inother embodiments, the plurality of copper nanoparticles maintain atleast some degree of porosity after being at least partially fused.

In other various embodiments, methods for fusing copper nanoparticles ina matrix material are disclosed herein. In some embodiments, methods ofthe present disclosure include providing a plurality of coppernanoparticles that are less than about 20 nm in size, mixing theplurality of copper nanoparticles with a matrix material and applying atleast one of heat or pressure to at least partially fuse the coppernanoparticles together. In some embodiments, the plurality of coppernanoparticles form a thermally conductive percolation pathway afterbeing at least partially fused. In some embodiments, the plurality ofcopper nanoparticles are substantially non-porous after being at leastpartially fused. In other embodiments, the plurality of coppernanoparticles maintain at least some degree of porosity after being atleast partially fused.

In some embodiments, the methods further include removing the matrixmaterial to leave behind a network of at least partially fused coppernanoparticles. Removal of the matrix material can take place by anyknown method including, for example, melting, dissolving, pyrolyzing,vaporizing, chemically reacting, and the like. For example, in anembodiment, the matrix material can be dissolved in a medium in whichthe matrix material is soluble but the copper nanoparticles aresubstantially insoluble and/or substantially non-reactive in their atleast partially fused state.

In some embodiments, the copper nanoparticles further include asurfactant system such as one of those described hereinabove. In someembodiments, the surfactant system includes a bidentate diamine (e.g., aC1-C4 dialkylethylenediamine, a C1-C4 N,N′-dialkylmethylenediamine or aC1-C4 N,N′-dialkyl-1,3-propylenediamine) and one or more C6-C18alkylamines. As noted above, other bidentate, tridentate or polydentateligands can also be used within the spirit and scope of the presentdisclosure. Likewise, alkylamines other than C6-C18 alkylamines can alsobe used in some embodiments.

In some embodiments of the present methods, the plurality of coppernanoparticles are between about 1 nm and about 10 nm in size. In otherembodiments, the plurality of copper nanoparticles are between about 1nm and about 5 nm in size or between about 5 nm and about 10 nm in size.In still other embodiments, the plurality of copper nanoparticles arebetween about 1 nm and about 20 nm in size. As noted above, the fusiontemperature of the copper nanoparticles will also depend upon theapplied pressure, in addition to the copper nanoparticle size. In someembodiments, the plurality of copper nanoparticles become at leastpartially fused together by heating at a temperature of at most about200° C. In other embodiments, the plurality of copper nanoparticlesbecome at least partially fused together by heating at a temperature ofat most about 100° C. Fusion temperatures of less than about 200° C. canbe particularly advantageous when thermally sensitive matrix materialsare employed.

In some embodiments, extruding the matrix material and the coppernanoparticles can result in at least partially fusing the coppernanoparticles together. In this case, extrusion forces can exertsufficient pressure on the copper nanoparticles so as to facilitatetheir fusion during the extrusion process.

In some embodiments, methods of the present disclosure further includecuring the matrix material. For example, the matrix material can be atwo-component epoxy in some embodiments, which is cured into a thermosetepoxy matrix. As another non-limiting example, the matrix material canbe a powder material that is sintered into a cured matrix material.Curing of the matrix material can take place concurrently with thefusion of the copper nanoparticles in some embodiments. Alternately, thematrix material can be cured prior to fusion of the copper nanoparticlesor after the fusion of the copper nanoparticles.

In still other various embodiments, methods are described herein forusing copper nanoparticles to join materials together. In variousembodiments, the methods include providing a plurality of coppernanoparticles that are mixed with a matrix material to form a paste,placing the paste in a joint between a first member and a second member,and joining the first member to the second member by at least partiallyfusing the plurality of copper nanoparticles together. The plurality ofcopper nanoparticles are less than about 20 nm in size and furthercontain a surfactant system of a bidentate diamine and one or moreC6-C18 alkylamines. As noted above other bidentate, tridentate orpolydentate ligands and alkylamines also lie within the spirit and scopeof the present disclosure.

Processes for joining materials together using copper nanoparticlescontained in a matrix material are particularly beneficial in the art.Specifically, the present methods complement processes in whichconventional tin- and lead-based soldering materials cannot beeffectively used. Further, the present methods allow two members to bejoined together at low temperatures that are generally not damaging tomost structural members.

A further advantage of the present methods is that, unlike conventionalsoldering techniques, copper nanoparticle-based soldering materials ofthe present disclosure can be used to join two non-metallic memberstogether or to join a non-metallic member to a metallic member. Likeconventional soldering techniques, the present methods can also be usedto join two metallic members together as well. Without being bound bytheory or mechanism, it is believed that inclusion of the matrixmaterial with the copper nanoparticles beneficially increases thecompatibility of the soldering material with a wide variety of materialsto achieve more effective joining and greatly increases thermal heattransfer compared to currently used solders.

In general, high concentrations of copper nanoparticles in a paste areused for joining two members together. In some embodiments, the pastecontains about 50% or higher copper nanoparticles by weight. In someembodiments, the paste contains about 60% or higher copper nanoparticlesby weight. In some embodiments, the paste contains about 70% or highercopper nanoparticles by weight. In some embodiments, the paste containsabout 80% or higher copper nanoparticles by weight. In some embodiments,the paste contains about 90% or higher copper nanoparticles by weight.

In applications for joining two members together, the plurality ofcopper nanoparticles can again be substantially non-porous or maintainat least some degree of porosity after being at least partially fused.In applications for joining two members together, maintaining at leastsome degree of porosity can allow for rework of the joint to take placeafter joining the first member to the second member. Rework can allowreplacement of a failed component from the joint, for example. In coppernanoparticle-based soldering applications not utilizing a matrixmaterial, maintaining porosity sufficient for rework can ultimately bedetrimental due to rapid copper nanoparticle oxidation, in addition torequiring much higher rework temperatures. However, inclusion of thematrix material in the present embodiments beneficially protects thecopper nanoparticles from oxidation while maintaining sufficientporosity to allow for rework. In some embodiments, the plurality ofcopper nanoparticles are up to about 25% porous after being at leastpartially fused together. In such embodiments, the copper nanoparticlescan maintain a sufficient tensile strength to maintain the first memberand the second member in a joined state. In some embodiments, a strengthto failure joining the first member to the second member is at leastabout 4400 psi.

In various embodiments, the present disclosure describes compositionsincluding a plurality of copper nanoparticles that are less than about20 nm in size and further contain a surfactant system having a bidentatediamine and one or more C6-C18 alkylamines, and a matrix materialselected from the group consisting of a polymer matrix, a rubber matrix,a ceramic matrix, a metal matrix, and a glass matrix. Such compositionscan be used in various applications described herein including, forexample, forming articles and joining a first member to a second member.

It is understood that modifications which do not substantially affectthe activity of the various embodiments of this invention are alsoincluded within the definition of the invention provided herein.Accordingly, the following Examples are intended to illustrate but notlimit the present invention.

EXPERIMENTAL EXAMPLES Example 1 Synthesis and Characterization of CopperNanoparticles

0.8 g of copper (II) chloride dihydrate was placed into a 250 mL 3 neckround bottom flask, evacuated and back-filled with argon three times. Aplastic syringe and a stainless steel needle were used to add thefollowing surfactants and organic solvents in the order given and underpositive argon pressure: 4 mL of N,N′-di-tert-butylethylenediamine, 4 mLn-nonylamine and 45 mL degassed triglyme. The color of the solutionturned dark blue. The copper salt solution was then stirred and heatedfor 2 hours at 45° C.

A second 250 mL 3 neck round bottom flask was charged with 8 mL of dry2.0 M sodium borohydride solution in triglyme. A plastic syringe and astainless steel needle were used to add the following surfactants in theorder given and under positive argon pressure: 4 mL of n-nonylamine and6 mL of N,N′-di-tert-butylethylenediamine. The resulting solution wascolorless. The reducing agent solution was heated to 45° C. and stirredfor 1 hour, during which time the solution turned slightly cloudy.

While keeping both flasks at 40° C., the copper (II) chloride solutionwas transferred to the flask containing the sodium borohydride reducingagent over a period of five minutes using a cannula. When the transferwas complete, the reaction mixture was dark purple. In the 10 minutesafter transfer, the reaction mixture turned cloudy and stayed white forabout 4 minutes. Subsequently, the reaction mixture kept changing colorfrom white, to yellow-brown and eventually dark brown over the next fiveminutes. At this point, the reaction mixture was cooled to −10° C. in adry ice/acetone bath for 10 minutes.

After cooling, the reaction mixture was centrifuged at 2000-3000 RPM for20 minutes, which resulted in a dark brown precipitate and a clearsupernatant. The black precipitate was mixed with a solution of degassedhexane (80 mL) and dicyclohexylamine (10 mL), which was then sonicatedfor 10 minutes. The new mixture was centrifuged at 2000-3000 RPM for 10minutes, and a black precipitate and clear supernatant were obtained.The black precipitate was mixed with a solution of degassed toluene (80mL) and dicyclohexylamine (10 mL), which was then sonicated andcentrifuged at 2000-3000 RPM for 10 minutes. A black precipitate and aclear supernatant were obtained. In the next step, 40 mL of degasseddeionized water was added to the black precipitate as well as 40 mLtoluene and 5 mL dicyclohexylamine. The mixture was shaken and sonicatedfor 10 minutes. The addition of the deionized water caused some gasevolution and bubbling. Thereafter, the mixture was sonicated andcentrifuged at 2000-3000 RPM for 20 minutes, which resulted in 3 layers:a dark brown organic (top layer), a second slightly cloudy aqueous layer(middle layer) and a small amount of a dark brown/copper colorprecipitate (bottom layer). The organic layer and the precipitate wereisolated and stored in a glass vial under argon at −5° C.

Scanning Electron Microscope Analysis: A JEOL JSM7001-FLV ScanningElectron Microscope (SEM) was employed in this analysis. FIG. 1 shows anillustrative SEM image of copper nanoparticles. Samples were preparedusing a dilute organic solution and TEM grids (Cu/Au, carbon coated,200/300 grid). The specified resolution was 1.2 nm at 30 kV and 3 nm at1 kV, and the magnification range was from 20× to 1,000,000×. The SEMalso offers a rapid chemical analysis using Energy DispersiveSpectroscopy (EDS). FIG. 2 shows an illustrative EDS spectrum of coppernanoparticles, which shows copper from the nanoparticles and carbon fromthe surfactant and/or solvent washes. The EDS also indicated a smallquantity of oxygen, possibly from oxidation arising from surfactantremoval in the SEM vacuum chamber.

X-Ray Diffraction (XRD) Analysis: A Siemens D5000 Diffractometer wasused for analysis. The dark precipitate of copper nanoparticles wasisolated and dried in air on a watch glass. A double sided sticky tape(0.5 cm×1 cm) was placed in the center of a standard glass slide. Thedark powder was placed on the tape and pressed down for good adhesionsuch that it covered the entire tape. The glass slide was placed in theXRD sample holder and the run conducted using the following conditions:Range 30°-80°, Step size: 0.1, Dwell time: 12, Deg: 5, Theta: 10°, LaserVoltage (kV) 40 and Current (mA) 30, run time 96 minutes. FIG. 3 showsan illustrative XRD spectrum of copper nanoparticles. The XRD spectrumindicated the presence of copper metal only, with no copper salt orcopper oxide detected.

Example 2 Fusion of Copper Nanoparticles

Copper nanoparticles produced as above were centrifuged for 20-30minutes at approximately 10,000 rpm such that a pressure of ˜3.9atmospheres was exerted on the copper nanoparticles. Under theseconditions, formation of micron-size copper crystals was observed. FIG.4 shows an illustrative SEM image of a micron-size copper crystal formedby fusion of copper nanoparticles at room temperature duringcentrifugation.

Upon heating, application of pressure, and/or extrusion for a sufficientlength of time, the copper nanoparticles can become substantially fusedtogether. FIGS. 5A and 5B show illustrative SEM images of a network ofsubstantially fused copper nanoparticles. FIG. 6 shows an illustrativeclose up SEM image of partially fused copper nanoparticles, whichdemonstrates widespread necking of the individual nanoparticles. FIG. 7shows an illustrative EDS spectrum of substantially fused coppernanoparticles. In this case, carbon from the surfactant and/or solventwashes was not observed in the EDS spectrum.

Although the invention has been described with reference to thedisclosed embodiments, those skilled in the art will readily appreciatethat these only illustrative of the invention. It should be understoodthat various modifications can be made without departing from the spiritof the invention, which is defined by the following claims.

What is claimed is the following:
 1. An article comprising: a matrixmaterial selected from the group consisting of a glass matrix, a ceramicmatrix, and a metal matrix; and a plurality of copper nanoparticles inthe matrix material that have been at least partially fused together toform a nanoparticle network; wherein the plurality of coppernanoparticles are less than about 20 nm in size.
 2. The article of claim1, wherein the plurality of copper nanoparticles further comprise asurfactant system.
 3. The article of claim 2, wherein the surfactantsystem comprises a bidentate diamine and one or more C6-C18 alkylamines.4. The article of claim 1, wherein the plurality of copper nanoparticlesrange between about 1 nm and about 10 nm in size.
 5. The article ofclaim 1, wherein the plurality of copper nanoparticles range betweenabout 1 nm and about 5 nm in size.
 6. The article of claim 1, furthercomprising: a filler material.
 7. The article of claim 6, wherein thefiller material is selected from the group consisting of flameretardants, UV protective agents, antioxidants, graphite, graphiteoxide, graphene, carbon nanotubes, fiber materials, ceramic materials,and combinations thereof.
 8. The article of claim 1, wherein the articlecomprises about 10% to about 99.9% copper nanoparticles.
 9. The articleof claim 1, wherein the article has a thermal conductivity rangingbetween about 50 watts/m·K and about 400 watts/m·K.
 10. A methodcomprising: providing a plurality of copper nanoparticles; wherein theplurality of copper nanoparticles are less than about 20 nm in size;mixing the plurality of copper nanoparticles with a matrix material; andapplying pressure to at least partially fuse the plurality of coppernanoparticles together to form a nanoparticle network.
 11. The method ofclaim 10, wherein the plurality of copper nanoparticles further comprisea surfactant system.
 12. The method of claim 11, wherein the surfactantsystem comprises a bidentate diamine and one or more C6-C18 alkylamines.13. The method of claim 10, wherein the plurality of coppernanoparticles range between about 1 nm and about 10 nm in size.
 14. Themethod of claim 10, wherein the plurality of copper nanoparticles rangebetween about 1 nm and about 5 nm in size.
 15. The method of claim 10,further comprising: curing the matrix material.
 16. The method of claim10, further comprising: removing the matrix material to leave behind thenanoparticle network, the nanoparticle network comprising a network ofat least partially fused copper nanoparticles.
 17. The method of claim10, wherein applying pressure comprises extruding a mixture of coppernanoparticles and the matrix material.
 18. The method of claim 10,wherein applying pressure comprises inkjet printing a mixture of coppernanoparticles and the matrix material.
 19. The method of claim 10,wherein applying pressure comprises press molding a mixture of coppernanoparticles and the matrix material.
 20. The method of claim 10,wherein the matrix material is selected from the group consisting of apolymer matrix, a rubber matrix, a glass matrix, a ceramic matrix and ametal matrix.